Livestock Science 155 (2013) 205–213
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Livestock Science
journal homepage: www.elsevier.com/locate/livsci
Daily and alternate day supplementation of urea or soybean
meal to ruminants consuming low-quality cool-season forage:
I—Effects on efficiency of nitrogen use and
nutrient digestion$, $$, %
D.L. McGuire a, D.W. Bohnert a,n, C.S. Schauer b, S.J. Falck c, R.F. Cooke a
a
Eastern Oregon Agricultural Research Center, Oregon State University, Burns, OR 97720, USA
Hettinger Research Extension Center, North Dakota State University, Hettinger, ND 58639, USA
c
Eastern Oregon Agricultural Research Center, USDA-ARS, Burns, OR 97720, USA
b
a r t i c l e i n f o
abstract
Article history:
Received 13 December 2012
Received in revised form
1 May 2013
Accepted 2 May 2013
Five Rambouillet Polypay wethers (527 2 kg BW; Experiment 1) and 5 Angus Hereford steers (464 7 26 kg BW; Experiment 2) were used in two incomplete 5 4 Latin
squares with four 18-d periods to determine the influence of supplemental N source and
supplementation frequency (SF) on efficiency of N use, nutrient intake, and nutrient
digestion in ruminants consuming hard fescue straw (4.7% CP). Wethers and steers were
provided straw at 120% of the previous 5 d average intake in two equal portions at 0730 h
and 1900 h. Treatments (TRT) included an unsupplemented control (CON) and a urea (29%
CP) or soybean meal (SBM; 26% CP) supplement provided daily (D) or every-other-day
(2D) at 0700 h. In Experiment 1, supplemental CP was provided at 0.10% of BW daily and
0.20% of BW every-other-day for D and 2D supplemented wethers, respectively. Feces and
urine were collected on d 13–18 for calculation of N balance and blood samples were
obtained 4 h post-supplementation on d 13–18 for analysis of plasma urea-N (PUN). In
Experiment 2, D TRT were supplemented CP at 0.04% of BW/day while 2D TRT received
0.08% of BW every-other-day. Feces were collected on d 13–18 for estimation of nutrient
digestibility. Dry matter intake, OM intake, N intake, N retention, DM, OM, and N
digestibility, and digested N retained were greater (Po 0.01) for supplemented wethers
compared with CON with no differences (P 40.05) because of N source or SF. There were
no differences in fecal or urinary N excretion because of supplementation, SF, or N source
(P 40.10). However, PUN was increased (P o 0.01) in supplemented lambs compared with
CON, whereas urea TRT had greater (P o 0.01) PUN compared with SBM. Plasma urea-N
was also increased (P ¼ 0.05) for D compared with 2D TRT. Straw and total DM and OM
intake were greater (P≤0.02) for supplemented steers compared with CON; however, DM
and OM digestibility was not influenced (P≥0.25) because of supplementation or SF. These
results suggest that supplements containing urea or SBM as the supplemental N source
Keywords:
Forage
Frequency
Non-protein N
Soybean meal
Supplementation
Urea
☆
The Eastern Oregon Agricultural Research Center, including the Burns and Union Stations, is jointly funded by the Oregon Agricultural Experiment
Station and USDA-Agricultural Research Service.
☆☆
Mention of trade names or commercial products in this publication is solely for the purpose of providing specific information and does not imply
recommendation or endorsement by the U.S. Department of Agriculture.
★
USDA is an equal opportunity provider and employer.
n
Correspondence to: Eastern Oregon Agricultural Research Center—Burns Station, Oregon State University, 67826-A Hwy 205, Burns, OR 97720, USA.
Tel.: +1 541 573 8910; fax: +1 541 573 3042.
E-mail address: dave.bohnert@oregonstate.edu (D.W. Bohnert).
1871-1413/$ - see front matter & 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.livsci.2013.05.015
206
D.L. McGuire et al. / Livestock Science 155 (2013) 205–213
can be used by lambs and steers consuming cool-season, low-quality forage without
adversely affecting N efficiency, nutrient intake, or nutrient digestibility, even when
provided every-other-day.
& 2013 Elsevier B.V. All rights reserved.
1. Introduction
Supplementation of natural protein to ruminants consuming low-quality forage (o6% CP) improved forage
intake (Bandyk et al., 2001; DelCurto et al., 1990; Köster
et al., 1996), nutrient digestibility (Bohnert et al., 2002a,
2002b; DelCurto et al., 1990), animal performance
(Bohnert et al., 2002b; Clanton and Zimmerman, 1970),
and reproductive efficiency (Sasser et al., 1988; Wiley et al.,
1991) compared with non-supplemented controls. Due to
its lower cost per unit of N compared with most sources of
natural protein, urea (non-protein N; NPN) is a popular
source of supplemental N. As with natural protein, providing supplemental urea to ruminants consuming lowquality forage increased forage intake (Egan, 1965;
Romero et al., 1976; Tudor and Morris, 1971), nutrient
digestibility (Ammerman et al., 1972; Briggs et al., 1947;
Raleigh and Wallace, 1963), and animal performance
(Currier et al., 2004a; Raleigh and Wallace, 1963;
Redman et al., 1980) when compared with no supplemental N. Nevertheless, compared with sources of natural
protein, NPN is often considered inferior as a supplemental
N source for ruminants consuming low-quality forage
(Clanton, 1978; Kropp et al., 1977); primarily because
of it lack of amino acid N and metabolizable protein.
However, when provided to mature ruminants consuming
low-quality forage at a level that does not exceed the
estimated requirement to maximize ruminal microbial
protein synthesis (NRC, 1996), NPN and natural protein
appear to be comparable as sources of supplemental N
(Cooke and Arthington, 2008; Farmer et al., 2004)
Infrequent (every-other-day to once every 7 d) supplementation of CP to ruminants consuming low-quality
forage can decrease the costs associated with supplemen-
tation while maintaining acceptable performance, nutrient
intake, and nutrient utilization when compared with daily
supplementation (Bohnert et al., 2002a, 2002b; Currier
et al., 2004a, 2004b; Huston et al., 1999a, 1999b). However,
infrequent supplementation of urea to ruminants is not a
common management practice because of the potential for
NH3 toxicity and decreased supplement intake (Chalupa,
1968; Helmer and Bartley, 1971). Consequently, there is
limited information available concerning infrequent supplementation of NPN to ruminants consuming low-quality
forage. Therefore, we conducted two experiments to
compare daily and every-other-day supplementation of
urea or soybean meal (SBM) on forage intake, nutrient
digestibility, and N efficiency in ruminants consuming
low-quality forage.
2. Materials and methods
The experimental protocol was approved by the Institutional Animal Care and Use Committee at Oregon State
University.
2.1. Experiment 1: N balance study
Five wethers (5272 kg) were used in an incomplete
5 4 Latin square design (Cochran and Cox, 1957) to
evaluate the efficiency of N use in lambs offered supplements in which urea or SBM provided the primary source
of supplemental N (Table 1). Wethers were allotted randomly to treatments and housed in individual metabolism
crates within an enclosed barn with continuous lighting.
Wethers had continuous access to fresh water and
chopped (4- to 8-cm length) hard fescue (Festuca
Table 1
Ingredient and nutrient content of hard fescue straw and supplements.
Item
Lamb study (Experiment 1)
Steer study (Experiment 2)
Hard fescue straw Urea supplement Soybean meal supplement Hard fescue straw Urea supplement Soybean meal supplement
Supplement composition (%)
Urea
–
Soybean meal –
Soybean hulls –
Dried molasses –
5.3
–
91.0
3.7
–
31.0
65.3
3.7
–
–
–
–
5.3
–
91.0
3.7
–
31.0
65.3
3.7
Nutrient composition
CP, % DM
4.7
DIPa, % CP
76.0
OM, % DM
93.3
NDF, % DM
76.8
ADF, % DM
40.0
29.2
83.0
94.7
58.6
40.2
26.4
76.4
93.8
45.8
30.0
4.7
76.0
93.2
78.4
40.6
29.0
83.0
94.8
59.0
40.3
26.4
76.4
94.1
45.6
29.1
a
Degradable intake protein. Estimates are based on dacron bag degradabilities. Techniques were similar to those described by Mass et al. (1999) and
Bohnert et al. (1998) for straw and supplements, respectively.
D.L. McGuire et al. / Livestock Science 155 (2013) 205–213
trachyphylla) straw (Table 1). Treatments consisted of an
unsupplemented control and a urea or SBM supplement
provided daily (D) or every-other-day (2D; CON¼control,
UD¼urea supplement every day, U2D¼urea supplement
every-other-day, SBMD¼SBM supplement every day, and
SBM2D¼SBM supplement every-other-day). Supplements
were offered D or 2D at 0700 h. The urea and SBM
treatments received the same amount of total supplemental N over a 2-d period; therefore, the 2D treatments
received double the quantity of supplemental N on their
respective supplementation day compared with D treatments. The amount of CP supplied by each supplement
was approximately 0.10% of BW/d (3.5 g supplement DM/
kg BW; averaged over a 2-d period). To minimize bias
because of potential BW changes resulting from treatment
regimes during each period, the quantity of supplement
provided in each period was based on initial BW at the
beginning of the experiment. Forage was provided daily at
120% of the average intake for the previous 5 d in two
equal portions (0730 h and 1900 h), with feed refusals
from the previous day determined before the 0700 h
feeding. Also, 35 g of a trace mineral salt mix (2.4% Ca,
2.3% P, 20.4% Na, 31.65% Cl, 0.2% K, 0.4% Mg, 0.1% S,
1309 ppm Mn, 2046 ppm Fe, 7 ppm Cu, 1930 ppm Zn,
42 ppm Co, 120 ppm I, 16 ppm Se, 1325 IU/kg Vitamin E,
and 552 and 50 kIU/kg Vitamins A and D, respectively) was
provided daily to each lamb at 0700 h. In addition, an
intramuscular injection of vitamins A, D, and E (200,000,
20,000, and 600 IU of Vitamins A, D, and E, respectively;
Vitamin E-AD 300; AgriLabs; St. Joseph, MO) was administered to each lamb at the onset of the trial to safeguard
against deficiency.
Experimental periods were 18 d with at least 3 d
between periods to allow for removal of wethers from
metabolism crates. Dry matter intake was determined on d
11–16. Samples of hard fescue straw and N supplements
(approximately 150 g/d) were collected on d 11–16 while
orts were collected and subsampled (20% of total daily
refusals; as-fed basis) on d 12–17. Samples of feed and orts
were dried at 55 1C for 48 h. On d 13–18, total fecal and
urine output was collected. Sufficient 6 N HCl (100 mL)
was added daily to urinals to maintain urine pH o3 to
minimize bacterial growth and N loss. This was verified
with pH paper during the urine collection period. Urine
was composited daily by wether (50% of total daily output;
weight basis) and stored at 4 1C. A sub-sample of each total
daily fecal sample (7.5%; wet weight basis) was dried at
55 1C for 96 h for calculation of fecal DM and then
composited by lamb within period. On d 13–18, 12 mL of
blood was collected via jugular venipuncture 4 h after the
0730 h straw feeding using a heparinized syringe. Blood
samples were immediately transferred to vacutainers
(Fisher Scientific, catalog no. 0268360), placed on ice for
transport to the lab, centrifuged (5000g for 15 min; 4 1C),
and plasma harvested and stored at −20 1C.
Dried samples were ground through a Wiley mill
(1-mm screen). Samples of ground hard fescue straw and
N supplements were composited by period and daily orts
composited by lamb (within period). Feed, orts, and fecal
samples were analyzed for DM, OM (AOAC, 1990), NDF
(Robertson and Van Soest, 1981) and ADF (Goering and
207
Van Soest, 1970) using procedures modified for use in an
Ankom 200 Fiber Analyzer (Ankom Co., Fairport, NY). Feed,
orts, fecal, and urine samples were analyzed for N using a
Leco CN-2000 (Leco Corporation, St. Joseph, MI). Plasma
samples were assayed for urea-N (PUNN) using the Sigma
Diagnostics Procedure 535 (Sigma Chemical Co., St. Louis,
MO) and a UV/vis spectrophotometer (Spectronic 710
Spectrophotometer, Bausch & Lomb, Inc., Rochester, NY).
2.2. Experiment 2: digestion study
Five Angus Hereford steers (464 726 kg) with ruminal cannulas were allotted randomly to 1 of 5 treatments
in an incomplete 5 4 Latin square (Cochran and Cox,
1957) and housed in individual pens (4 8 m) within an
enclosed barn with continuous lighting. Treatments were
the same as described in Experiment 1. Urea treatments
were formulated to provide 100% of the estimated degradable intake protein (DIP) requirement assuming a microbial efficiency of 11% (NRC, 1996; Model 1) while SBM
treatments were provided on an isonitrogenous basis with
urea treatments. Nitrogen supplements were placed
directly into the rumen via the ruminal cannula at
0700 h for supplemented treatments. The D TRT were
supplemented CP at 0.04% of BW/day (1.3 g DM/kg BW)
while the 2D TRT were supplemented at 0.08% of BW
(2.6 g DM/kg BW) every-other-day. As in Experiment 1, the
quantity of supplement provided in each period was based
on initial BW at the beginning of the experiment to
minimize potential bias due to BW changes that could
result from the different treatment regimens during each
period. Steers had continuous access to fresh water and
chopped (4- to 8-cm length) hard fescue straw. Nutrient
content of the hard fescue straw and N supplements is
listed in Table 1. Estimates of DIP were determined based
on in situ degradability using techniques similar to those
described by Bohnert et al. (1998) and Mass et al. (1999)
for supplements and hard fescue straw, respectively. Straw
was provided daily at 120% of the previous 5 d average
intake in two equal portions (0730 h and 1900 h), with
feed refusals from the previous day determined before
feeding. A trace mineralized salt mix was available free
choice (7.3% Ca, 7.2% P, 27.8% Na, 23.1% Cl, 1.5% K, 1.7% Mg,
0.5% S, 2307 ppm Mn, 3034 ppm Fe, 1340 ppm Cu,
3202 ppm Zn, 32 ppm Co, 78 ppm I, 90 ppm Se, 79 IU/kg
vitamin E, and 397 kIU/kg vitamin A). In addition, an
intramuscular injection of vitamins A, D, and E (500,000,
50,000, and 1500 IU of Vitamins A, D, and E, respectively;
Vitamin E-AD 300; AgriLabs; St. Joseph, MO) was administered to each steer at the onset of the trial to safeguard
against deficiency.
Experimental periods were 18 d, with 9 d of diet
adaptation and 9 d of sampling. Intake was measured
beginning d 11 and concluded d 16 (subsamples of straw
and supplements, approximately 150 g/d as-fed, were
collected at this time) while orts were measured and
subsampled (5% of total daily refusal; as-fed basis) on d
12–17.
On d 11 and 12, treatment effects on ruminal particulate fill were determined by manually removing reticulorumen contents 4 h after feeding. This allowed sampling
208
D.L. McGuire et al. / Livestock Science 155 (2013) 205–213
when D and 2D supplements were provided and when
only D supplements were provided. Total ruminal contents
were weighed, mixed by hand, and sub-sampled in triplicate (approximately 400 g/triplicate). The remaining ruminal contents were replaced immediately into the animal.
Ruminal samples were weighed, dried in a forced-air oven
(55 1C; 96 h), reweighed in order to calculate DM, ground
to pass a 1-mm screen in a Wiley mill, and composited
within period and day by steer. A more complete description of these procedures is provided in a companion paper
(Cappellozza et al., 2013).
Feces were collected on d 13–18. Steers were fitted with
harnesses and fecal bags on d 13 at 0730 h. Fecal bags were
weighed and emptied twice daily at 0730 h and 1630 h.
Feces collected at 1630 h were stored individually by steer
in a sealed 190-L polyethylene bag for mixing with the
0730 h collection the following morning (24-h fecal
collection). Feces were manually mixed and a 2.5% subsample (wet weight) obtained, dried for 96 h at 55 1C,
re-weighed for DM, and composited by steer within
period. Dried samples of straw, orts, and feces were
ground through a Wiley mill (1-mm screen).
Ground samples of hard fescue straw and N supplements were composited by period and daily orts composited by steer (within period) on an equal weight basis.
Feed, orts, and feces were analyzed for DM, OM, N, NDF,
and ADF as described in Experiment 1.
3. Statistical analysis
For all analyses, results are reported as LS Means,
significance was set at P≤0.05 and tendencies were determined if P 40.05 and ≤0.10. Results are reported according
to treatment effects if no interactions were significant or
according to highest-order interaction detected.
3.1. Experiment 1: N balance study
Data were analyzed as an incomplete 5 4 Latin square
(Cochran and Cox, 1957) using the MIXED procedure of
SAS (SAS Inst. Inc., Cary, NC) and Satterwaite approximation to determine the denominator degrees of freedom for
the test of fixed effects. The model included treatment and
period as independent variables. Wether was used as the
random variable. Because the treatment structure consisted of a 2 2 factorial plus a negative control, orthogonal contrasts were used to partition specific treatment
effects. Contrast statements included (1) CON vs. supplementation; (2) urea vs. SBM; (3) D vs. 2D supplementation; and (4) N source SF. The model used for PUN
included treatment, day, and treatment day interaction,
and period as the independent variable. Wether was used
as the random variable. The specified term for the repeated
statement was day, lamb (period treatment) was
included as the subject, and autoregressive was used as
the covariance structure based on it providing the lowest
Akaike information criterion. The same contrasts noted
above were used to partition treatment sums of squares.
3.2. Experiment 2: digestion study
Data were analyzed as an incomplete 5 4 Latin square
using the MIXED procedure of SAS and Satterwaite
approximation to determine the denominator degrees of
freedom for the test of fixed effects. The model included
treatment and period as independent variables. Steer was
used as a random variable. The same orthogonal contrasts
used in Experiment 1 were used to partition specific
treatment effects.
4. Results
4.1. Experiment 1: N balance study
Straw DM and OM intake by wethers were not affected
by supplementation, N source, or SF (P≥0.15; Table 2).
Conversely, total DM and OM intake were increased with
supplementation (P o0.01), but not by N source or SF
(P≥0.15). We noted a tendency for NDF intake to increase
(P¼0.07) with supplementation, but it was not influenced
by N source or SF (P≥0.14). Nitrogen intake was increased
(Po0.01) with supplementation and tended (P ¼0.06) to
be greater for urea supplemented steers than for SBM
steers; however, SF had no affect on N intake (P≥0.35).
Apparent total tract digestibility of DM, OM, ADF, and N
were increased (Po0.01) with supplementation and not
affected (P≥0.22) by SF (Table 2). However, urea supplemented wethers had lower total tract OM (P ¼0.05)
digestibility and tended to have lower total tract DM
(P¼0.08) digestibility compared with SBM supplemented
wethers. Total tract ADF and N digestibility were not
influenced by N source (P≥0.55). Apparent total tract
NDF digestibility tended to be greater with supplementation (P¼ 0.06) and daily supplementation (P¼0.10), but
was not influenced (P¼ 0.82) by N source.
Fecal and urinary N excretion were not altered (P≥0.16)
by supplementation, N source, or SF (Table 2). In contrast,
N balance and digested N retained were increased
(Po0.01) with supplementation while not affected by N
source or SF (P≥0.40).
Treatment day interactions (Po0.01) were observed
for PUN. However, after considering the nature of the
interactions, we concluded that discussing treatment
means while providing the treatment day figure (Fig. 1)
would aid in interpretation and discussion of the data.
Plasma urea-N was increased with supplementation
(Po0.01), greater for urea compared with SBM
(Po0.01), and greater for D compared with 2D (P ¼0.05;
Table 2; Fig. 1).
4.2. Experiment 2: digestion study
Supplemented steers had greater straw and total DM
and OM intake (P≤0.02) compared with CON steers and
consequently, N and NDF intake also increased with
supplementation (P o0.01). Conversely, none of these
parameters were further affected by N source or SF
(P≥0.28; Table 3).
Apparent total tract DM and OM digestibility were not
affected by supplementation (P≥0.33), for urea compared
D.L. McGuire et al. / Livestock Science 155 (2013) 205–213
209
Table 2
Effects of supplemental N source and supplementation frequency on intake, diet digestibility, and N balance in lambs consuming low-quality forage.
Item
Treatmenta
CON
P-valuec
UD
U2D
SBMD
SBM2D
SEMb
Con vs. Supp
Urea vs. SBM
D vs. 2D
N source SF
Daily DM intake, g/kg BW
Straw
Supplementd
Total
23.2
0.0
23.2
23.2
3.5
26.7
21.5
3.5
25.0
22.5
3.5
26.0
22.3
3.5
25.8
1.52
0.27
0.94
0.16
0.28
1.52
o 0.01
0.93
0.16
0.28
Daily OM intake, g/kg BW
Straw
Supplemente
Total
21.6
0.0
21.6
21.7
3.3
25.0
20.0
3.3
23.3
21.0
3.3
24.3
20.8
3.3
24.1
1.42
0.29
0.92
0.15
0.26
1.42
o 0.01
0.95
0.15
0.26
Daily NDF intake, g/kg BW
17.8
19.8
18.3
18.7
18.7
1.15
0.07
0.46
0.14
0.18
0.0121
o 0.01
0.06
0.35
0.73
0.93
0.80
1.02
2.09
2.38
o 0.01
o 0.01
0.06
o 0.01
o 0.01
0.08
0.05
0.82
0.57
0.55
0.56
0.28
0.10
0.22
0.34
0.95
0.85
0.79
0.52
0.79
0.0112
0.0178
0.81
0.38
0.16
0.27
0.25
0.93
0.88
0.33
Daily N intake, g/kg BW
0.176
Apparent total tract digestibility, %
DM
37.9
OM
40.5
NDF
42.7
ADF
31.7
N
14.9
Daily N excretion, g/kg BW
Fecal
Urinary
Daily N balance, g/kg BW
Daily digested N retainedf, %
Plasma urea-N, mM
0.151
0.070
−0.042
0.330
42.4
45.4
45.9
45.3
49.3
0.165
0.087
0.327
42.0
44.8
43.8
43.9
53.1
0.154
0.107
0.321
43.9
47.0
45.9
45.4
53.0
0.151
0.085
0.314
43.4
46.1
44.3
41.3
54.8
0.142
0.069
o 0.01
0.40
0.99
0.52
−178.3
46.4
35.7
42.5
57.5
40.48
o 0.01
0.83
0.96
0.76
3.7
7.0
6.1
5.0
4.9
0.22
o 0.01
o 0.01
0.05
0.15
0.080
0.064
0.085
0.101
0.0245
a
CON¼ control; UD ¼urea supplement every day; U2D ¼ urea supplement every-other-day; SBMD ¼soybean meal supplement every day;
SBM2D ¼soybean meal supplement every-other-day.
b
n¼ 4.
c
Con vs. Supp¼ control vs. supplemented treatments; Urea vs. SBM ¼urea vs. soybean meal treatments; D vs. 2D ¼daily vs. alternate day
supplementation; N Source SF ¼interaction of N source vs. supplementation frequency.
d
UD and SBMD received 3.47 g/kg BW daily; U2D and SBM2D received 6.94 g/kg BW every-other-day.
e
UD received 3.29 g/kg BW daily; U2D received 6.58 g/kg BW every-other-day; SBMD received 3.25 g/kg BW daily; SBM2D received 6.50 g/kg BW
every-other-day.
f
Calculated as (Daily N retention, g/kg BW/Daily N digested, g/kg BW) 100.
source or SF. Total tract NDF digestibility was not influenced by supplementation, N source, or SF (P≥0.11).
5. Discussion
Fig. 1. Effects of supplemental N source and supplementation frequency
(SF) on lamb plasma urea-N (mM) 4 h post-feeding. Left and right
columns for each treatment represent when all supplements were
offered and when only daily supplements were offered, respectively.
Treatments were: control; UD ¼ urea supplement every day; U2D ¼urea
supplement every-other-day; SBMD ¼ soybean meal supplement every
day; SBM2D¼ soybean meal supplement every-other-day. Treatment SF
interaction is (Po 0.01). SEM for treatment SF is 0.25.
with SBM supplementation (P≥0.51), or for D compared
with 2D supplementation (P≥0.25; Table 3). In contrast,
apparent total tract N digestibility was increased (Po0.01)
with supplementation and not affected (P≥0.19) by N
The primary goal of the present study was to investigate the effects of supplementation frequency (daily or
every-other-day) and supplemental N source (SBM or
urea) on forage intake, nutrient digestibility and N efficiency in ruminants consuming low-quality cool-season
forage (o5% CP; DM basis). Infrequent CP supplementation has been shown to be a viable option to reduce costs
associated with supplementation (such as labor, fuel and
feed) while maintaining adequate performance in cattle
(Bohnert et al., 2002a, 2002b; Currier et al., 2004a, 2004b;
Huston et al., 1999a,1999b).
In agreement with previous research (DelCurto et al.,
1990; Köster et al., 1996), straw DM and OM intake were
increased by supplementation in steers (Experiment 2);
however, N source and SF had no effect. The observed
increase in forage intake with supplementation suggests
that ruminal N may have been limiting in unsupplemented
steers. In a companion paper, Cappellozza et al. (2013)
210
D.L. McGuire et al. / Livestock Science 155 (2013) 205–213
Table 3
Effects of supplemental N source and supplementation frequency on nutrient intake and disappearance in steers.
Item
Treatmenta
P-valuec
CON
UD
U2D
SBMD
SBM2D
SEMb
Con vs. Supp
Urea vs. SBM
D vs. 2D
N source SF
Daily DM intake, g/kg BW
Straw
Supplementd
Total
15.7
0.0
15.7
17.1
1.3
18.4
17.3
1.3
18.6
17.6
1.3
18.9
17.0
1.3
18.3
0.88
0.02
0.90
0.68
0.37
0.88
o 0.01
0.90
0.68
0.37
Daily OM intake, g/kg BW
Straw
Supplemente
Total
14.6
0.0
14.6
16.0
1.2
17.2
16.2
1.2
17.4
16.4
1.2
17.6
15.9
1.2
17.1
0.83
0.02
0.89
0.66
0.38
0.83
0.02
0.89
0.66
0.38
Daily NDF intake, g/kg BW
12.4
14.2
14.4
14.4
13.9
0.71
o 0.01
0.79
0.64
0.37
0.006
o 0.01
0.28
0.88
0.59
1.3
1.3
1.3
1.7
0.33
0.34
0.99
o 0.01
0.51
0.61
0.37
0.19
0.31
0.25
0.11
0.31
0.26
0.29
0.73
0.06
Daily N intake, g/kg BW
Apparent total-tract digestibility, %
DM
OM
NDF
N
0.12
51.3
53.4
56.1
28.5
0.19
51.7
53.7
55.5
43.5
0.19
54.2
56.2
57.7
48.9
0.19
52.3
54.3
54.8
44.7
0.18
52.2
54.4
56.4
42.9
a
CON¼ control; UD ¼ urea supplement every day; U2D¼ urea supplement every-other-day; SBMD ¼ soybean meal supplement every day;
SBM2D ¼ soybean meal supplement every-other-day.
b
n¼ 4.
c
Con vs Supp¼ control vs. supplemented treatments; Urea vs. SBM ¼ urea vs. soybean meal treatments; D vs. 2D ¼ daily vs. alternate day
supplementation; N source SF ¼ interaction of N source vs. supplementation frequency.
d
UD and SBMD received 1.30 g/kg BW daily; U2D and SBM2D received 2.60 g/kg BW every-other-day.
e
UD received 1.23 g/kg BW daily; U2D received 2.46 g/kg BW every-other-day; SBMD received 1.22 g/kg BW daily; SBM2D received 2.44 g/kg BW
every-other-day.
reported average ruminal NH3-N concentrations of
1.32 mM in unsupplemented steers. This concentration is
near the lower end of the levels shown to be needed for
maximal growth of ruminal microbes in vivo (1.18–
2.94 mM; Slyter et al., 1979). Also, ruminal IADF passage
rate was increased with supplementation (Cappellozza
et al., 2013). Therefore increased ruminally available N
may have resulted in increased microbial growth and the
subsequent increase in straw intake.
Conversely, straw DM and OM intake in lambs were not
influenced by supplementation, N source or SF. These
findings are in agreement with results from Currier et al.
(2004a), who reported no effects of N supplementation on
straw DM and OM intake in a similarly designed study. It
has been suggested that DMI is maximized and will not
respond to supplementation when daily NDF intake is
roughly ≥12.5 g/kg BW (Mertens, 1994). In the current
study, lamb daily NDF intake was 17.8 g/kg BW for the CON
group and ranged from 18.3 to 19.8 g/kg BW in supplemented lambs. Thus, forage intake was not expected to
increase. This coincides with other work related to a lack
of forage intake following N supplementation of lowquality cool-season forages (Bohnert et al., 2002a, 2002b;
Currier et al., 2004a, 2004b; Schauer et al., 2010). It should
be noted that NDF intake increased in Experiments 1 and
2, likely a result of NDF provided by the supplements
(which ranged from 46% to 59%; Table 1). Also, the lack of a
supplementation effect on straw intake in Experiment 1
may have resulted from adequate protein intake without
supplementation. Preston et al. (1965) reported that a PUN
in excess of 3.57 mM indicates adequate protein intake for
lambs consuming high-fiber based diets. In the current
study, CON lambs had a PUN concentration of 3.7 mM
suggesting that protein intake may have been adequate
prior to supplementation. Conversely, the negative N
balance and digested N retained with CON along with
improvement in N balance and digested N retained with
supplementation suggest that lambs were deficient in N
prior to supplementation.
Protein supplementation has been shown to increase
total tract digestibility in ruminants consuming lowquality forage (Bohnert et al., 2002a, 2002b; Currier
et al., 2004c; Wickersham et al., 2008b). As expected,
supplementation increased DM, OM, ADF and N digestibility, and tended to increase NDF digestibility, in lambs.
Conversely, N supplementation, regardless of source, failed
to increase DM, OM, and NDF digestibility in steers.
Similarly, Wickersham et al. (2008b) reported that increasing amounts of DIP (casein) had no effect on total tract
digestibility of OM and NDF in steers. The authors suggest
that the lack of a supplementation effect may have been
caused by an increased intake and passage rate in response
to increased ruminally available N supply (Guthrie and
Wagner, 1988; Olson et al., 1999). This agrees with our
observation of increased straw intake as well as ruminal
IADF passage rate in supplemented steers from Experiment 2 reported by Cappellozza et al. (2013). As a result,
the length of time during which ruminal microbes had
access to the substrate was reduced, thereby, potentially
masking the positive effects on digestibility often observed
due to supplementation of low-quality forage.
Apparent total-tract N digestibility increased significantly with supplementation in both experiments. This is
in agreement with other results observed with ruminants
D.L. McGuire et al. / Livestock Science 155 (2013) 205–213
consuming low-quality forage and supplemented with CP
(Bohnert et al., 2002a, 2002b; Currier et al., 2004a; Ferrell
et al., 1999). The low N digestibility observed in the CON
groups (28.5% in steers and 14.9% in lambs) is probably a
result of the high fiber and low CP of the forage. Additionally, metabolic fecal N, the portion of fecal N that is not
directly from the diet (Strozinski and Chandler, 1972), can
constitute a significant portion of total fecal N in unsupplemented ruminants, resulting in low apparent N digestibility (Ferrell et al., 1999) due to the relatively consistent
quantity of metabolic fecal N (5.35 g N/kg DMI; NRC 1985)
and low N intake in ruminants consuming low-quality
forages (Currier et al., 2004a). The greater N digestibility
with supplementation is likely due to the greater digestibility of supplements compared to low-quality forage and
decreased metabolic fecal N as a percentage of N intake
(Currier et al., 2004a; Wickersham et al., 2008a).
The increased N balance and digested N retained
observed with supplementation agrees with previous
research in which supplemental N was provided to ruminants consuming low-quality forage (Bohnert et al., 2002b;
Currier et al., 2004a; Egan, 1965). The lack of a N source
effect agrees with previous work by Ammerman et al.
(1972), who reported no differences in N retention or
balance between lambs supplemented with either SBM
or an NPN source. Also, infrequent CP supplementation of
ruminants consuming low-quality forages has resulted in
similar N balance and digested N retained compared with
daily supplementation (Bohnert et al., 2002b; Currier et al.,
2004a; Romero et al., 1976). Supporting our results,
Romero et al. (1976) supplemented steers consuming 2%
CP forage with an oral drench of urea twice daily, once
daily or once every 2 d. Nitrogen balance was −5.0 g/d for
unsupplemented steers and increased to 7.0, 5.7 and 5.7 g/d
for twice daily, once daily and once every 2 d, respectively.
Supplementation frequency did not affect N balance.
Similarly, Bohnert et al. (2002b) supplemented wethers
consuming 5.2% CP meadow hay with low- or high-DIP
supplements daily, once every 3 d, or once every 6 d and
increased N balance compared to an unsupplemented
control. However, the authors reported a linear decrease
in N balance as SF decreased, which they attributed to a
similar decrease in N intake as SF decreased which was
supported by no effect of SF on digested N retained. These
findings suggest that infrequent N supplementation to
ruminants consuming low-quality forage has minimal
effects on N balance and digested N retained compared
with daily supplementation.
Plasma urea concentration has been shown to be
positively correlated with N intake (Harmeyer and
Martens, 1980). Likewise, we observed increases of 83%
and 55% in N intake and PUN, respectively, with supplementation compared with the CON. Other research has
shown increased PUN with CP supplementation of lowquality forages (Bohnert et al., 2002b; Ferrell et al., 1999;
Krehbiel et al., 1998). An effect of N source on PUN was
also observed, with increased PUN in lambs supplemented
with urea compared with SBM. Urea is quickly hydrolyzed
to NH3-N in the rumen, which is then either utilized by the
ruminal microbes, absorbed across the rumen wall, or
flows to the duodenum for absorption in the small
211
intestine. Ammonia absorbed from the gastrointestinal
tract is rapidly converted to urea by the liver (Van Soest,
1982) and subsequently released into the circulation.
While a portion of supplemental N from SBM is converted
to NH3-N in the rumen, a lower, delayed peak in ruminal
NH3-N concentrations is typically observed when compared to NPN sources (Owens and Zinn, 1988). Consequently, lower ruminal NH3-N levels, and thus lower PUN
concentrations, should be expected with SBM supplementation when compared to urea supplementation.
Decreased PUN has been reported with decreased SF in
other studies with ruminants consuming foraged-based
diets (Bohnert et al., 2002b; Currier et al., 2004a; Huston
et al., 1999a). In general, the response to infrequent
supplementation is a larger peak in PUN following supplementation events compared with daily treatments. This
typically occurs within 24 h and is proportional to the
quantity of supplement provided (Currier et al., 2004a;
Schauer et al., 2010). We noted an almost 100% decrease in
PUN on the day after supplementation for the urea 2D
treatment (Fig. 1). This response for urea 2D is similar to
the response reported by Currier et al. (2004a) for lambs
consuming the same treatment and comparable quality
forage. In contrast, we noted a small increase in PUN for
SBM 2D compared with D, most likely a consequence of
delayed ruminal/intestinal digestion of protein in SBM. As
a result, the decreased average PUN observed with the 2D
compared with D treatments appears primarily due to the
aforementioned decrease in PUN for urea 2D on the day
following supplementation (Fig. 1).
6. Conclusion
In conclusion, supplementation of steers consuming
low-quality forage increased total and forage DMI compared with unsupplemented steers. Conversely, supplementation had no influence on DMI by lambs, likely a
result of high NDF intake, greater N intake as a percentage
of BW, and sufficient ruminal available N. Additionally, N
source and supplementation frequency had little effect on
intake and digestibility of low-quality hard fescue straw by
lambs and steers. These results suggest that supplements
containing urea or SBM as the supplemental N source can
be effectively used by lambs and steers consuming coolseason, low-quality forage without adversely affecting N
efficiency, nutrient intake, or nutrient digestibility, even
when provided every-other-day. As a result, reducing
supplementation frequency to every-other-day would
result in a 50% savings in the costs associated with
supplementation (fuel, labor, etc.). Also, supplementation
costs can be further reduced by incorporating urea, normally cheaper per unit N than sources of natural protein,
into supplementation programs for use by mature ruminants consuming low–quality forages.
Conflict of interest
None.
212
D.L. McGuire et al. / Livestock Science 155 (2013) 205–213
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